168 
HISTORY OF BRITISH FERNS. 
little risk of frosts, for the least frost would destroy them, 
and, indeed, it is not uncommon for the earlier growth to 
be destroyed in exposed places by the very slight frosts 
which occur at that season of the year. 
The fronds themselves have been variously described, 
and often erroneously, for they are not unfrequently said 
to be three-branched, a form which really occurs in one of 
the smaller Polypodies (P. Dryopteris), They are not 
properly three-branched, and except when very much 
starved and stunted, do not approach that form very nearly. 
They are, in reality, bipinnate, or, when very luxuriant, tri- 
pinnate, the pinnae standing opposite in pairs, each pair in 
succession becoming fully developed, while the main rachis 
is extending upwards, and the next pair is beginning to un- 
fold. The mature fronds are thus truly bi- or tri-pinnate, 
with the pairs of pinnae standing opposite. When the fronds 
are much diminished in size by the sterility of the soil 
which sustains them, they become almost triangular, and 
then have somewhat the appearance of a three-branched 
frond, the development of the lower pair of branches not 
leaving the plant energy enough to carry up its rachis, 
and produce the other pairs of pinnae which it would 
normally possess. That this is the true habit of the 
